1. Field of the Invention
The present invention relates to a packet detection system utilized in a multi-band orthogonal frequency division multiplexing system, and more particularly, to a packet detection unit of a packet detection system set to correspond to different time frequency codes.
2. Description of the Prior Art
In recent years, wireless communication technology and devices have taken a big leap with advances in research and development. More specifically in the field of wireless personal area network (WPAN), the WPAN technique allows users to establish personal wireless transmitting spaces with a longest diameter of 10 meters. Nowadays, the two major WPAN techniques are respectively “Bluetooth” and infrared transmission. “Bluetooth” utilizes EM waves, which are hardly blocked by any obstacle, to transmit data while the infrared transmission can achieve high-speed transmission within a certain distance such as one meter. These technological advances have made obtaining information, and communication between people significantly easier and faster. For this reason, the development of wireless personal area network is commonly recognized as one of the most significant technologies of network communication.
Typically, Inter Symbol Interference (ISI) is largely caused by a limited bandwidth in high-speed data transmission, producing interference in the transmitted data. In wireless communication technology, an orthogonal frequency division multiplexing system is capable of utilizing a simpler frequency domain equalizer to replace a more complicated time domain equalizer. Therefore, orthogonal frequency division multiplexing technology is widely utilized in every advanced digital communication system including: digital video broadcast, ultra wideband technology, and so forth.
Ultra wideband (UWB) systems incorporate a multi-band orthogonal frequency division multiplexing (MB-OFDM) technology. The MB-OFDM method begins by separating a frequency spectrum into several sub-bands. Each sub-band bandwidth length is 4.125 MHz. Each domain orthogonal frequency division multiplexing (OFMD) symbol is transmitted via a time-interleaved method across the sub-band. With reference to FIG. 1, please note that the signal transmission will be limited to a sub-frequency band at any given time. The main advantage of utilizing the time-interleaved method is that an average transmitted power is the same as a system designed to operate over the entire bandwidth, and information is processed over much smaller bandwidth (˜500 MHz). Thus usage elasticity of the frequency spectrum increases, as does compatibility with world telecommunication regulations.
In order to realize the advantage of the above system, the receiving end of the MB-OFDM system is required to be fast and accurate in order to select the appropriate timing and frequency band (which is time synchronization and frequency synchronization). Therefore, the receiving end of a packet detection device is crucial.
FIG. 2 illustrates a diagram of a conventional packet detection device 20. The operation of the packet detection device 20 is well known to those skilled in the art and thus it does not need to be further explained. The receiving end of a conventional communication system is capable of determining whether a received signal r(n) is data or noise via the packet detection device 20. When data is received, an output signal M(n) of the packet detection device 20 is close to a high-level signal. On the other hand, when noise is received (White Gaussian Noise (WGN) for example), the output signal M(n) will be close to a low-level signal.
The receiving end of the conventional communication system is capable of determining whether a data packet is received via the packet detection device 20. However, the packet detection device 20 is not operable with the ultra wideband wireless communication system utilizing the MB-OFDM technology for the reasons listed below:
1. Frequency hopping: the MB-OFDM technology utilizes the frequency hopping method such that a frequency band utilized by a symbol is different from the frequency band utilized by a previous symbol. This protocol prevents a traditional packet detection device 20 from detecting an incoming packet accurately. The frequency hopping method refers to the transmission end, which will switch to different carrier frequency band to transmit signals. Therefore only a predetermined receiving end can receive signals accurately. For other receiving ends, the hopping signals generated by the frequency hopping technology are merely noise.
2. Time frequency code: To allow the receiving end to accurately receive signals, the transmission end sets the time frequency code at the front end of an output signal for describing timing, and the receiving end selects different timing and frequency hopping sequences according to different time frequency codes. However, the traditional packet detection device 20 cannot detect the corresponding packet due to different time frequency codes. Therefore the traditional packet detection device 20 is not applicable on the UWB wireless communication system utilizing MB-OFDM technology.
3. Timing detection issue: when the transmission end of the MF-OFDM system utilizes a zero-padding prefix, the receiving end cannot obtain accurate timing information. This prevents the packet detection device 20 from determining a timing position of a fast Fourier transform (FFT) sampling window.
In regards to the timing detection issue mentioned above, please refer to the following explanation. For the traditional orthogonal frequency division multiplexing system as shown in FIG. 3, after processing a baseband signal in the transmission end, the data will first be transmitted to an Inverse Fast Fourier Transform (IFFT) where the tail part of a symbol will then be prefixed to the beginning of the symbol (IFFT o/p). The prefixed part is called a cyclic prefix. On the contrary, the cyclic prefix is removed from the symbol passing through a channel and the symbol will be received by the FFT. When there is ISI caused by limited bandwidth, the cyclic prefix maintains the FFT sub-carrier of the FFT window. Therefore, the effect created by the ISI can now be eliminated. However, when the zero-padding prefix is employed in the MB-OFDM system (as shown in FIG. 4), the conventional packet detector cannot determine the precise FFT window position thus the processing of the signal afterwards will be affected.
Therefore in order to realize high-speed transmission through a multi-band orthogonal frequency division multiplexing system, design of the receiving end of the packet detection device is a very relevant issue.